The invention relates to a temperature control device for an NMR sample tube, wherein multiple interleaved, concentric flow channels for temperature control fluid extending coaxially with respect to a cylindrical interior space for holding the NMR sample tube are constituted around said interior space,
wherein said temperature control device is constituted such that it is closed toward the interior space in an axial end region and, in an axial end region at the opposite end thereto, open to the interior space for inserting the NMR sample tube into said interior space,wherein, in a counter flow region, adjacent flow channels are interconnected through fluid passage at one axial end in such a way that the direction of a fluid flow in the flow channels of the counter flow region is reversed with respect to the corresponding adjacent flow channel in the counter flow region,wherein the outermost flow channel of the counter flow region has a fluid inlet at one axial end,and wherein the innermost flow channel of the counter flow region has a fluid passage to the interior space at one axial end that faces toward the axial end region that is closed.
Such a temperature control device is known from DE 40 18 734 C2.
Nuclear magnetic resonance (NMR) spectroscopy is a powerful method of instrumental analysis. In NMR spectroscopy, radio-frequency (RF) pulses are irradiated into a measurement sample that is disposed in a strong, static magnetic field, and the RF response of the measurement sample is then measured. The information is obtained by integration across a certain region of the measurement sample, termed the active volume.
The temperature of the measurement sample always influences the result of an NMR spectroscopy measurement. To obtain high-quality measurement results, the measurement sample is kept at a temperature that is as constant and as uniform as possible across the entire active volume. To this end, a temperature control device is typically located in the sample head of an NMR spectrometer. Heating and cooling measurement samples for NMR measurement are both common practice.
In a common measurement method, the measurement sample (that is, the substance from which analytical information is to be derived) is disposed in an NMR sample tube. Typical NMR sample tubes have an outside diameter of 5 mm and a length of approx. 18 cm, wherein the active volume extends along approx. 25 mm of the length. In the NMR spectrometer, the NMR sample tube is held at one (usually the upper) end and the other, free (usually lower) end is surrounded by RF coils and protrudes into the temperature control device; frequently, the RF coils are integrated in the temperature control device.
In common temperature control devices, a temperature-controlled fluid (usually a gas such as air or nitrogen, in some cases, also a liquid) flows around the sample tube at least in the region of the active volume. In the simplest case, the temperature control device constitutes a chamber into which a free end of the sample tube is inserted through an opening; a fluid flow is then introduced into the chamber and directed toward the free end. The fluid exits in the region of the opening (which is not completely blocked at the edge of the sample tube). However, such simple temperature control devices exhibit relatively high temperature gradients in the fluid flow and/or along the length of the active volume of the NMR sample tube because the temperature of the fluid flow comes closer to the ambient temperature as it passes through the chamber. (For example, it cools down in the case of a heating temperature-controlling fluid flow.) Heat flows, in particular, through the outer wall of the temperature control device through which the fluid is flowing and also along the NMR sample tube itself toward the tube holder (mounting).
To reduce the temperature gradient in the fluid flow surrounding the NMR sample tube, it is known that temperature control devices can be constituted according to the counter flow principle, cf. DE 40 18 734 C2, FIG. 2 therein. In DE 40 18 734 C2, a fluid flow is initially conveyed along an outer, ring-shaped flow channel in a first axial direction parallel to the sample tube, then redirected and conveyed back in the opposite axial direction along an inner flow channel and then redirected again and conveyed directly along the sample tube and out through the opening into which the NMR sample tube protrudes into the interior space of the temperature control device. The walls between the flow channels and toward the interior space act as heat exchangers, so that the temperature gradient in the fluid flow immediately adjacent to the sample tube is noticeably reduced.
One disadvantage of this prior art, however, is the relatively low temperature control performance that can be achieved by the fluid flow. The multiple redirection and the routing of the fluid flow immediately adjacently to the sample tube causes a large flow resistance, which limits the fluid flow. There is also a danger of the NMR sample tube being pressed out of the temperature control device and damaged under high fluid pressure.
The object of the invention is to ensure temperature control of an NMR sample tube with low temperature gradients while achieving a high temperature control performance.